1. Field of the Invention
The invention relates to an apparatus and method for printing upon a recording medium and more particularly to an apparatus for printing permanent images electrographically upon two sides of a paper medium at comparatively high speeds as is required in a computer print-out apparatus.
2. Description of the Prior Art
It had long been recognized that computer peripherals, particularly computer print-out apparatus, were bottlenecks in the total performance of a computer system. The majority of hard-copy output devices for computer systems were and still are comprised of printers which impact the paper medium with print hammers. The movement of such print hammers not only limits the speed at which read-out can be accomplished but are noisy and difficult to maintain. In order to increase the speed, facilitate maintenance, and still maintain print quality, a system was developed and is now being marketed commercially by Honeywell Information Systems Inc., utilizing electrographic techniques to accomplish non-impact printing. Such a printing system which prints electrographically on one side only of a pre-treated paper medium is disclosed in the above cited U.S. Pat. No. 3,687,107, issued Aug. 29, 1972. That patent teaches how to print on one side of a recording medium by exciting the recording medium with energy corresponding to the shapes to be printed, developing it and drying, by squeezing the paper between two surfaces at least one of which is absorbent, and then scraping the absorbent surface so as to render it absorptive again.
Another U.S. Patent also issued to Honeywell Inc. on Nov. 30, 1971 having U.S. Pat. No. 3,624,661 pertains to an electrographic printing system, having a multiple electrode structure wherein successive rows are mutually spaced from each other, each row including mutually spaced electrodes, the electrodes of successive rows being positioned in a staggered manner with respect to each other.
U.S. Pat. No. 3,958,251 discloses an electrographic printer provided with multiple row electrode structure wherein the electrodes in each row are mutually spaced one from the other and the electrodes of successive rows are staggered with respect to one another.
U.S. Pat. No. 3,812,780 discloses an electrographic printing device provided with a forms print station including an electrode drum having forms information raised therein which operates on a dielectric print medium.
U.S. Pat. No. 3,839,071 discloses a method of printing wherein a latent image is first formed on a recording medium and thereafter developed by applying a toning liquid to the recording medium.
U.S. Pat. No. 3,983,815 discloses a method and apparatus for electrographically printing on a dielectric paper and transferring a toned image from the dielectric paper to plain paper.
U.S. Pat. No. 3,934,112 discloses an electrographic printing system with improved apparatus and method used to evaporate a volatile carrier and to deposit toner particles on a printing medium, and to fix the deposited colored toner particles permanently to the paper medium.
The above-cited articles of Paragraphs 3 and 4, on page 2, also describe the non-impact page printing system now being marketed by Honeywell Information Systems Inc.
The above system significantly increased the printing speed from approximately 1110 lines per minute for a high speed impact printer to approximately 18,000 lines per minute for the non-impact page printer.
Further improvements in number of lines per minute printed without increasing speed of travel of the recording medium, can be effected by printing simultaneously on both sides of the recording medium. However, in order to successfully do this without sacrificing print quality, several problems must be overcome. One problem is the application and retention of electric changes on the recording medium without breakdown of the air gap between electrodes on opposite sides of the medium. For example, with the printing of variable information on any side of the recording medium, it is generally required to place formatting information on the recording medium. When using only one side of the recording medium, this is relatively simple since the voltage breakdown of the air gap is not exceeded. However, when two sides of the recording medium are utilized for printing, the total applied voltage between format drum and electrode assembly can exceed the air breakdown voltage and spark over because of the two electrodes on opposite sides of the paper. Yet too little voltage will not provide for requisite print quality. Accordingly, relative spacing and size of format drums and electrodes, and relative thickness of dielectric as well as total thickness of the recording medium is of utmost importance.
Another problem is the matter of developing and fixing the latent images on two sides of the recording medium instead of one. These and other problems encountered do not offer trivial solutions.